Slag is a versatile yet demanding material in the context of demolition, deconstruction, cutting, and separation works. It arises primarily from metallurgical processes, thermal cutting and welding, and as a combustion residue. In buildings and industrial plants, slag may occur as a binder constituent, as aggregate, as fill or base-layer material, as well as an adhesion on steel components. For planning concrete demolition and deconstruction, strip-out and cutting operations, it is therefore important to understand the properties of slags and assess their influence on methods, tools, and occupational safety. There is particular practical relevance wherever concrete demolition shears or hydraulic rock and concrete splitters are used to separate components in a material-appropriate way and prepare them as single-material fractions for recycling.
Definition: What is meant by slag
Slag refers to predominantly inorganic mixtures of oxides and silicates that solidify as glassy to crystalline materials and arise as by-products or residues of industrial high-temperature processes. These include in particular blast furnace slag (smelter slag), steelworks and foundry slags, welding and cutting slags, as well as incineration slags from waste or power plants. In binder technology, granulated blast-furnace slag is called Hüttensand; when ground and finely distributed, it acts as a latent hydraulic component and is used in cement technology. Depending on origin, slag can show variable compositions (e.g., CaO, SiO₂, Al₂O₃, MgO, FeO/Fe₂O₃, MnO, sulfides) and variable particle sizes. Legally, slag can be classified as a by-product or as waste depending on its field of use and origin; evaluation follows applicable technical rules and regional requirements.
Formation and types of slag
Slag forms when, at high temperatures, unwanted accompanying substances, oxides, and silicates melt, separate from metals or fuels, and then solidify. The result ranges from porous, lightweight slags to dense, glassy materials with high abrasiveness. For demolition, cutting, and recycling, it is critical whether the slag is brittle, glassy, metal-bearing, magnetic, or chemically reactive (e.g., free lime, sulfides). These properties influence tool selection, cutting path, target size reduction, and downstream sorting.
- Blast furnace slag (BFS): By-product of hot metal production; processed as granulated smelter slag for cement and as aggregate. Latently hydraulic, often light to greenish, glassy, with a homogeneous particle-size distribution.
- Steelworks and foundry slags: Heterogeneous, sometimes dense, dark pieces; may contain metallic inclusions, free CaO/MgO, and magnetic constituents. Suitability as construction material must be assessed case by case.
- Welding and cutting slags: Thin, brittle adhesions or droplets at cut edges; they influence rework, dimensional accuracy, and surface quality when dismantling steel structures.
- Incineration slags (grate slag, MSWI slag): Granular residues with varying glass and mineral phases; often contain metals (non-ferrous/ferrous) and must be processed for recovery.
- Non-ferrous metal slags: For example copper or nickel slags; sometimes used as blasting media, often dark, dense, and highly abrasive.
Slag in concrete, mortar, and recycled construction materials
Granulated blast-furnace slag (Hüttensand) is used as a grinding addition in cement or as aggregate. Slag-rich binder systems are characterized by dense microstructures, often good sulfate resistance, and reduced heat of hydration. In industrial floors and civil engineering works, slags can also occur as aggregate or in base layers. For concrete demolition this means: The matrix can be very dense and abrasive, potentially with increased density. This affects the crushing work of concrete demolition shears and wear. When using hydraulic wedge splitters, the crack path is often defined, while the required splitting forces vary depending on the microstructure.
Particularities when deconstructing slag-bearing concretes
- Density and abrasiveness may be increased; this influences tool wear and cutting speeds.
- Magnetic constituents (metal splinters) in slag aggregates can either facilitate or interfere with the separation of reinforcement and aggregates—depending on the separation stage and magnet technology.
- Free lime/MgO in certain slags may tend to volumetric change; during processing and storage, watch for potential swelling.
- The grading curve of recycled aggregates from slag-bearing concrete often requires careful post-screening to maintain a stable grading band.
Influence on demolition and separation methods
Whether in concrete demolition and special demolition or in strip-out and cutting, slag—either as an embedded constituent or as an adhesion—can determine the choice of method. Glassy, brittle slags tend to chip, while dense slags fail bluntly. This influences whether fracture-mechanical separation (hydraulic wedge splitters) or comminuting separation (e.g., concrete demolition shears) is advantageous. On steel components, cutting slag impairs fit-up and requires rework by shearing or by reapplying the cutting torch.
Tool selection and process chain
- Crushing and separating: Concrete demolition shears open the matrix, reduce component thicknesses, and expose reinforcement—even in dense, slag-influenced binders.
- Splitting and controlled crack management: Hydraulic wedge splitters generate defined separation cracks with low vibration—helpful in sensitive areas, for example at installations with dust-sensitive surroundings.
- Steel processing: Combination shears, multi cutters, and steel shears remove slag-laden edges and cut sections and reinforcement after concrete deconstruction.
- Thermal separation: When using a cutting torch, cutting slag is generated; cut orientation influences slag drip-off and subsequent rework.
- Power supply: Hydraulic power packs provide the flow rate for shears and split cylinders; with dense, abrasive materials, a reliable power reserve is important.
Practice in concrete demolition and special demolition
A robust process plan combines investigation, suitable separation steps, and consistent sorting. The aim is a high recycling rate and safe execution with minimized dust and noise exposure.
Investigation and sampling
- Review existing documentation: use of slag cement, slag-bearing base layers, industrial build-ups.
- Visual assessment: color, glass content, metallic inclusions, density indicators.
- Spot samples: grading curve, density, magnetic separability, and, if necessary, leachate and content values for preliminary classification.
Separation strategy
- Pre-crushing with concrete demolition shears to expose reinforcement and reduce cross-sections.
- Targeted splitting with hydraulic wedge splitters on load-bearing cores or in sensitive zones.
- Finish cutting and reinforcement separation using combination shears or steel shears; thermal cutting as needed.
- Sorting into fractions: concrete/slag, reinforcing steel, non-ferrous metals, fines.
Processing and quality
- Crushing and screening to achieve a defined particle-size distribution.
- Magnetic separation to increase purity; particularly effective with metal-bearing slags.
- Control fines and glass breakage to limit dust generation.
Slag in thermal cutting and steel dismantling
In oxy-fuel or plasma-assisted cutting, cutting slag forms from oxidized metal and fluxes. It solidifies at the underside of the cut or falls as dripping slag. For downstream dismantling, smooth, low-slag cut surfaces are advantageous: steel shears grip more cleanly, combination shears jam less. In confined spaces, such as during the tank dismantling, guiding slag is important to prevent accumulations that must later be removed mechanically.
Practical notes
- Select the cutting position so that slag can drip off in a controlled manner.
- Remove adhesions from cut surfaces before shearing to reduce blade wear.
- Protect surrounding components from sparks and slag deposits.
Safety, environment, and disposal
Slag can release fine dust, metal oxides, and potentially soluble constituents. When handling, minimize emissions and separate materials carefully. The notes are general and do not replace a case-by-case assessment.
- Dust and emissions: Plan suitable dust extraction, misting/wetting, and respiratory protection; especially with glassy fines and during crushing.
- Chemical aspects: Free CaO/MgO can react with moisture; during storage, watch for swelling and pH.
- Leaching: Reusability depends on material properties and regional limit values; plan sampling and documentation early.
- Thermal risks: Freshly generated cutting slag is hot and brittle; beware of injury from splinters.
Quality assurance and documentation
A transparent material balance facilitates recycling and disposal. For each fraction, origin, quantity, and quality should be recorded. This applies in particular to mixed fractions of concrete, slag, and reinforcement.
- Record grading curves, density, and magnetic fractions of the mineral fractions.
- Photo documentation of cut surfaces and adhesions before and after processing.
- Log the power units, operating pressures, and tools used to keep separation processes reproducible.
Fields of application and relation to the equipment of Darda GmbH
In concrete demolition and special demolition, concrete demolition shears help open dense, slag-influenced concretes and expose reinforcement. Hydraulic wedge splitters are suitable for generating controlled cracks in massive cores where vibrations must be avoided. In strip-out and cutting, slag-laden steel parts are reworked using combination shears, multi cutters, and steel shears; cutting torches are used for the thermal separation of tanks and pipeline sections, where cutting slag is unavoidable. Hydraulic power packs supply shears and split cylinders; reliable performance is particularly important with abrasive materials. In special applications—for example, in sensitive areas with strict emission protection—fracture-mechanical methods can offer advantages.
Terminology and typical misunderstandings
Slag is not the same as ash: ashes are usually finer and lighter, whereas slags are glassy to coarse-grained and often denser. Hüttensand (granulated blast-furnace slag) is not finished cement, but acts as a latently hydraulic addition in cement. Cutting slag is not an independent construction material, but an adhesion and waste product of thermal separation that should be mechanically removed before further processing.
Practical tips for planning and execution
- Clarify early whether slag-bearing binders or aggregates were installed; this influences wear, separation sequence, and sorting.
- Where possible, separate mechanically first (shears, splitting) and then follow up thermally; this reduces cutting slag.
- Limit fines and glass breakage through adapted size reduction; plan dust suppression.
- Provide magnetic separation as a standard stage wherever metallic impurities are expected.
- Keep fractions as clean as possible to safeguard recovery options for recycled construction materials.